The present invention relates to the field of semiconductor technology and, in particular, to a method for detaching layers or layer systems from a carrier substrate and subsequently applying them to an alternative substrate.
A detachment of a layer or a layer system from a carrier substrate is known. It is accomplished, for example, by a full chemical dissolution, as well as a mechanical or mechanical-chemical back-thinning of the substrate.
One of the objects of the present invention is to provide a method for detaching layers or layer systems while the substrate is preserved and the layers or layer systems can subsequently be used on other substrates.
This object is achieved with a method for detaching layers or layer systems from a substrate and subsequently applying them onto an alternative substrate. The method according to the present invention provides that, e.g.,
a) a porous breakaway layer is formed by anodizing in a hydrofluoric acid;
b) a stabilizing layer with a lower porosity is optionally formed over the breakaway layer;
c) oxide of the breakaway layer or the stabilizing layer is dissolved by a brief contact with the hydrofluoric acid (xe2x80x9cHFxe2x80x9d);
d) an epitaxial layer is applied on the breakaway layer or the stabilizing layer;
e) the epitaxial layer or the layer system composed of the breakaway layer and the stabilizing layer is detached from the substrate;
f) the epitaxial layer or the layer system is applied onto an alternative substrate; and
g) the stabilizing layer and/or the rest of the breakaway layer is detached from the epitaxial layer.
Thus, by producing a breakaway layer in a layer system, a part of this layer system can be detached from the substrate in a controlled manner.
Preferably, p-doped Si, SiC and/or Ge are used as the substrate and as the raw material for the breakaway layer and the stabilizing layer.
Porous silicon, the porous breakaway layer and the low-porosity support layer can be produced by an electrochemical anodizing procedure in a mixture containing hydrofluoric acid (e.g., ethanol). If silicon is used, for example, it is converted to SiO2 in the known manner, which is subsequently made to react with HF.
A porosity of the layers can be adjusted as a function of the current density, doping, and HF concentration.
Since the porosity determines the mechanical and chemical stability of the layer, it can be optimized in this manner. The size of the pores may be in the range of a few nm up to tens of nm. Porosity can also be adjusted in a range of 10% to over 90%. In general, porosity of the porous silicon layer can be increased by increasing current density, reducing HF concentration, and reducing doping. Increasing pore size allows the highly porous layer to be very quickly etched and dissolved selectively with respect to the substrate or a low-porosity layer.
High porosity of the breakaway layer can reduce mechanical stability to the point where the layer on top of the highly porous layer can be mechanically lifted off.
The breakaway layer can have a) a single porosity, b) a sequence of layers having different porosities, or c) a continuous increase and then decrease in the porosity, i.e., a porosity gradient. In the case of the single porosity, a detachment takes place in the breakaway layer or in the abrupt transitions from the low-porosity to the high-porosity layer. In the case of the sequence of layers, several high-porosity breakaway layers with abrupt porosity transitions are created at or in which the layer is ruptured. In the case of the continuous increase and then the decrease in the porosity, a rupture at the porosity transition areas is prevented by the continuous increase and decrease in the porosity, and thus it can only take place in the high-porosity part of the breakaway layer.
Therefore, an additional low-porosity layer made of the same material can be used to stabilize the breakaway layer. This is achieved during an anodization procedure, preferably using a lower current density.
Porous layer systems generally preserve their lattice spacing due to the manufacturing process. However, a lower-porosity layer can be advantageous for a subsequent epitaxial layer to be grown thereon. Thus, the lower-porosity layer not only contributes to stabilizing the entire layer system, but also makes a sufficient number of nuclei available for a possible subsequent epitaxial layer, allowing a high-quality epitaxial layer to grow.
In one embodiment of the method according to the present invention, the breakaway layer is stabilized by a partial oxidation.
This has an advantage that the stability of the layer system is increased in the subsequent separation steps, which are required for applying the epitaxial layer, and the subsequent tempering steps occurring at high temperatures. Prior to applying the epitaxial layer, a brief contact with HF is then required in order to remove the oxide of the layer supporting the epitaxial layer. This contact must be sufficiently short so that while the oxide of the upper low-porosity layer is removed allowing it to then be used as an epitaxial layer, the partly or fully oxidized porous breakaway layer is not yet fully removed.
In another embodiment of the method according to the present invention, a non-wet-chemical oxidation and a subsequent reaction with gaseous HF can be performed with the oxidation product after the partial oxidation.
An advantage of this embodiment is that, e.g., when the high-porosity silicon layers are dried, the resulting capillary forces may become so high that the layers are destroyed. Therefore, the risk of rupture existing with the wet chemical method can be avoided with this embodiment. Thus, the porous layers produced in the anodization remain stable even during the drying process and only lose their mechanical stability by the subsequent HF vapor treatment. In this process step, a loss of stability is desirable because it allows, e.g., layers grown epitaxially to be removed.
A loss of stability of porous silicon is a result of a volume loss and an increase in the internal surface area of silicon. The oxidation step used in this embodiment of the method according to the present invention can be performed by a storage in an oxygen atmosphere or in the air.
The epitaxial layer can be applied by depositing an amorphous layer from the gaseous phase on the breakaway layer or the stabilizing layer and subsequently recrystallizing this amorphous layer by a tempering procedure.
The amorphous layer is deposited from the gaseous phase in a conventional manner using technologies such as PECVE (plasma enhanced chemical vapor epitaxy), MOVPE (metal-organic vapor phase epitaxy), reactive or non-reactive sputtering or PECVD (plasma enhanced chemical vapor deposition).
The amorphous precipitate initially formed is converted to a crystalline form by tempering, the lattice spacing of the underlying layer system forming the basis of the epitaxial layer growth.
The layer grown can be tempered with the entire layer system, including the substrate. It is also possible to temper the layer grown after mechanically removing it. The monocrystalline porous stabilizing layer and thus the nucleus cells required for recrystallization are preserved due to the presence of the breakaway layer. It is also possible to perform a tempering procedure in part with and in part without the substrate.
The epitaxial layer or the layer system can be removed chemically, mechanically, or electrochemically.
Chemically, the porous silicon, for example, is removed using KOH, NaOH, NH3 or mixtures of hydrofluoric acid with oxidants such as H2O2 or HNO3.
To stabilize the layer to be removed, the upper layer system can be anodically bonded to glass or plastic. The removal process can then be accelerated using ultrasound. Other bonding or gluing methods allow the layer system to be attached to alternative substrates.
Furthermore, the removal can be accomplished electrochemically. This electrochemical removal is only possible when the entire layer system (e.g., including the epitaxial layer) is etched again electrochemically in the hydrofluoric acid. Normally, the hydrofluoric acid cannot penetrate the epitaxial layer to allow electrochemical etching, unless the epitaxial layer is previously structured with holes. In electrochemical removal, a very high current density results in the removal of the upper porous or structured non-porous layer, depending on doping, HF concentration, and the thickness of the breakaway layer.
The method according to the present invention allows layer systems to be advantageously removed from the original substrate and applied to alternative substrates such as glass, plastic or ceramic, or further processed.
A reusability of the substrate allows applications such as electronics on glass or plastic to be implemented in a simpler manner.
Solutions used to remove the breakaway layer can also be used for cleaning the epitaxial layer, i.e., to remove the stabilizing layer and residues of the breakaway layer. One of the objects of the present invention is to manufacture a cleaner removed epitaxial layer.